Polycarbonates, which comprise dihydric or polyhydric phenols linked by carbonyl groups, are often injection molded to form parts having low dimensional tolerances as well as excellent impact resistances over wide temperature ranges. Drawbacks to the use of these polymers include their low scratch and chemical resistances. For this reason, and for improved appearance, polycarbonate moldings are often coated with other materials having greater hardnesses or other desirable surface properties.
For example, Adams et al. U.S. Pat. No. 4,927,675 proposed a coextruded multilayer material comprising a thermoplastic core layer having at least one inorganic and/or organic constituent dispersed therein; a first coextruded thermoplastic outer layer attached to a first surface of the thermoplastic core layer and substantially free of dispersed inorganic and organic constituents; and a second coextruded outer layer attached to a second surface of the core layer opposite the first surface and substantially free of dispersed inorganic or organic constituents. The thermoplastic core layer was typically a polycarbonate, as were the first and second coextruded outer layers. A coating material, for example silicone polymer (that is, an organopolysiloxane polymer or copolymer), might be applied on the first and second outer layers after coextrusion of the multilayer material in those instances in which additional physical properties were desired, such as abrasion resistance. Adams et al. asserted that such multilayer materials overcame the defects of prior art monolayer materials which might damage extruders or auxiliary equipment and which might allow colorants and reinforcing agents to react unfavorably with subsequently applied dyes or inks.
Automobiles and other land vehicles are often displayed for sale in enclosed showrooms lit by fluorescent lights. Under such lighting conditions, unsightly optical interference fringes forming "rainbow" or "cat's eye" patterns have been observed on door sashes and cowl covers comprising injection molded black polycarbonate substrates with overlying 4-8 .mu.m silicone polymer coatings. These colored interference fringes may render such automobiles less attractive to potential purchasers. It is an object of the present invention to suppress the appearance of colored interference fringes on coated plastic parts.
Such interference patterns are believed to result from the wave nature of light. As is well known, light is an electromagnetic field whose strength varies periodically with both time and distance as the field moves past a given location. The field strength at any location is additive--that is, if two beams are moving through the same location at the same time in the same direction, the total field strength would be the sum of the field strength of each of the two beams at that location.
It is possible to characterize a light beam by imagining it as the combination of a series of coincident component beams, each of whose field strengths vary only for a certain amount of time characteristic of that component (referred to as a "period") before repeating themselves. Since the speed of light, that is, the speed at which the electromagnetic field is moving through a particular material, is a constant (equal to the speed of light in a vacuum divided by an "index of refraction" of the material), each such component beam moves through a characteristic distance (referred to as a "wavelength") during its period.
A component of light having a particular period and wavelength when moving through one material will have a different wavelength when moving through a different material. In fact, it is the product of multiplying the wavelength of the component in a particular material by the index of refraction of that material which remains constant. Thus, "optical distances," that is, the products of multiplying measured distances through materials by the indices of refraction of those materials, are often compared when studying the movement of light through laminate materials. Despite the changes in the values of the wavelengths as the components move from one material to another, the term "wavelengths" will be used as a shorthand for these components.
Each wavelength within the range of about 400-700 nm is perceived as a color. White light is a combination of many different wavelengths (that is, many different colors), all traveling in the same direction at the same time.
Ideally, the variation of the field strength for a given wavelength over time would be "sinusoidal" (that is, would correspond with a "sine function" such as that shown graphically at 10 in FIG. 1). The variation of the fields strength with distance in the direction in which the beam is moving also would be ideally sinusoidal. If two such ideal beams from different sources were combined, the sine function expressing the field strength for a particular wavelength in one beam may be delayed (that is, shifted to the right with respect to the axis 12 in FIG. 1) relative to the corresponding sine function for that wavelength in the other beam. Then, there is said to be a "phase difference" between the two components at that wavelength. If one of the two sine functions is delayed by half of the period 14 (that is, if they are "fully out of phase" as shown graphically at 10 and 16 in FIG. 1), the two components "destructively interfere" with each other and make no contribution to the intensity or color of the combined light beam.
Although applicants are not to be bound by any particular theory of operation of the invention, it is believed that the prior art door sashes and cowl covers produce colored interference fringes due to destructive interference between light partially reflected off the surface of the silicone polymer coating and light reflected from the interface between the black polycarbonate substrate and the coating. When white light impinges on the outer surface of the silicone polymer coating, the coating transmits a portion of the light toward the interface with the substrate and reflects the remainder. Next, the substrate absorbs some of the portion transmitted by the coating and reflects the rest of that portion back toward the outer surface of the silicone polymer coating. In particular, a black substrate will absorb or reflect all colors (that is, wavelengths) of light approximately equally, which promotes the formation of interference fringes. The light partially reflected from the outer surface of the coating and from the substrate then recombines on the outer surface of the coating and projects toward the viewer.
When the reflected light recombines, certain wavelengths of the light reflected off the substrate are approximately fully out of phase with the corresponding wavelengths in the light reflected off the coating because the light partially reflected off the substrate travels a greater distance than that partially reflected off the outer surface. These wavelengths undergo destructive interference and are removed from the light projected toward the observer. The observer sees bands or fringes of light tinted by the wavelengths remaining in the projected light.